1. Field of the Invention
The present invention relates to an active matrix substrate used in a display or the like and to a method for manufacturing the active matrix substrate.
2. Description of the Background Art
An active matrix substrate (hereinafter referred to as a “TFT substrate”) that includes a thin film transistor (TFT) serving as a switching element is widely known to be used in an electro-optical device such as a liquid crystal display (LCD). The LCD including the TFT substrate (TFT-LCD) requires improvements on display characteristics (such as a wide viewing angle, high definition, and high quality) and also requires low costs as a result of simplifying manufacturing steps for an efficient manufacturing.
The general TFT-LCD has a liquid crystal cell as the basic structure in which the TFT substrate (element substrate) and a counter substrate (CF substrate) sandwich a liquid crystal layer, and the TFT-LCD is formed of the liquid crystal cell including a polarizer bonded thereto. The TFT substrate includes a plurality of pixels in which a pixel electrode and a TFT that is connected to the pixel electrode are disposed in a matrix pattern. The counter substrate includes a color filter (CF) and a counter electrode disposed opposite to the pixel electrode. For example, a full transmissive LCD includes a backlight (BL) on a back surface side of the liquid crystal cell.
In this manner, the liquid crystal cell including the pixel electrode and the counter electrode for generating an electric field to drive the liquid crystals disposed so as to sandwich the liquid crystal layer is a liquid crystal cell of a vertical electric field driving method typified by a twisted nematic (TN) mode. The TFT substrate of the TN mode is normally manufactured in four or five photolithography steps (photolithography processes). For example, Japanese Patent Application Laid-Open No. 64-35529 (1989), Japanese Patent Application Laid-Open No. 2001-311965, and Japanese Patent Application Laid-Open No. 2009-25788 below disclose a manufacturing method for forming the TFT substrate in three photolithography steps.
On the other hand, in terms of the wide viewing angle of the TFT-LCD, an in-plane switching (IPS) mode (“IPS” is a trademark) has been developed, the IPS mode being one of lateral electric field driving methods including both of the pixel electrode and the counter electrode disposed on the TFT substrate. The IPS mode can obtain the viewing angle wider than that of the vertical electric field driving method, but an image display portion has an aperture ratio and a transmittance lower than those of the vertical electric field driving method, so that it is difficult to obtain bright display characteristics. This problem arises in the situation where an electric field for driving liquid crystals fails to operate effectively on liquid crystals in a region directly above the pixel electrode having a comb pattern. A fringe field switching (FFS) mode has been developed as a lateral electric field driving method capable of improving this problem (for example, Japanese Patent Application Laid-Open No. 2001-56474).
Moreover, in terms of high definition and high quality of the TFT-LCD, a technology has been developed to use an oxide semiconductor having a mobility higher than that of the conventional Si for a semiconductor serving as an active layer of the TFT formed in the TFT substrate (for example, Japanese Patent Application Laid-Open No. 2004-103957, Japanese Patent Application Laid-Open No. 2005-77822, Japanese Patent Application Laid-Open No. 2008-72011 and Nature Vol. 432 (2004) p. 488). Examples of the oxide semiconductor include a zinc oxide (ZnO) system and an InGaZnO system in which a gallium oxide (Ga2O3) and an indium oxide (In2O3) are added to the zinc oxide (ZnO). The oxide semiconductor film has light-transmissive properties higher than those of a Si semiconductor film, and Japanese Patent Application Laid-Open No. 2007-115902, for example, discloses that an oxide semiconductor film having a transmittance of greater than or equal to 70% to visible light of 400 nm to 800 nm.
The oxide semiconductor film above can be etched with a weak acid solution such as oxalic acid and carboxylic acid, and thus there is an advantage that a pattern is easily processed. However, the oxide semiconductor film is easily dissolved by an acid solution normally used in an etching process on general metal films (Cr, Ti, Mo, Ta, Al, Cu, and alloys thereof) that are used for a source electrode and a drain electrode of the TFT. Thus, when the metal films to be the source electrode and the drain electrode are etched (patterned), it is necessary to consider preventing the oxide semiconductor film from being destroyed. For example, Japanese Patent Application Laid-Open No. 2005-77822 discloses a technology to add a new element to the oxide semiconductor to improve resistance to a chemical solution and a technology to optimize film thicknesses of the metal film and the oxide semiconductor film to be the source electrode and the drain electrode.
The LCD of the FFS mode has excellent viewing angle characteristics and panel transmittance, whereby demand is on the increase. However, the TFT substrate used for the LCD of the FFS mode requires both of the pixel electrode and the counter electrode (common electrode) to be formed on the TFT substrate, thereby increasing the number of wire layers of the TFT substrate. This increases the number of photolithography steps required for forming the TFT substrate, which causes an increase in manufacturing costs.
For example, the TFT substrate of the general FFS-LCD disclosed in FIGS. 1 and 3 of Japanese Patent Application Laid-Open No. 2001-56474 is manufactured in six photolithography steps. As described above, the manufacturing method that requires the three photolithography steps for the TFT substrate of the conventional TN mode has been developed, and thus reducing the number of photolithography steps is a major challenge in manufacturing the TFT substrate of the FFS mode.
To solve the problem, Japanese Patent Application Laid-Open No. 2001-235763 and Japanese Patent Application Laid-Open No. 2009-157366 disclose a method to reduce the photolithography steps to four or five times in manufacturing the TFT substrate of the FFS mode. However, the number of photolithography steps is still great compared to the manufacture of the TFT substrate of the TN mode, whereby an increase in the manufacturing costs is inevitable.
Furthermore, as described above, the general oxide semiconductor film is easily dissolved by the acid solution used for etching the metal films (Cr, Ti, Mo, Ta, Al, Cu, and alloys thereof) that are used for the source electrode and the drain electrode of the TFT. The general oxide semiconductor is difficult to be used in a case of the structure exposing a lower layer of the semiconductor film upon the etching process on the source electrode and the drain electrode, as with the structure of the TFT in Japanese Patent Application Laid-Open No. 2001-56474 (FIGS. 1 and 3), Japanese Patent Application Laid-Open No. 2001-235763 (FIG. 3), and Japanese Patent Application Laid-Open No. 2009-157366 (FIG. 5).